The present invention relates to floor coverings and more particularly to durable tile or sheet form floor coverings made of one or more layers of polymers suitable for e.g. pedestrian traffic in domestic and/or other situations over an extended period of time.
Most floor coverings of this type are based on polyvinylchloride (PVC) polymer. In more detail, PVC polymer resin is generally mixed with a plasticiser (solid or liquid) (usually with various other additives such as fillers, polymer stabilisers, and processing aids) to form a spreadable paste which can be formed into sheets by spread coating using knife or roller coater equipment and then thermally cured e.g. by oven heating.
The use of PVC does however raise significant environmental problems due to the use of chlorine and there is accordingly a need for floor coverings based on alternative polymers. Polyalkene polymers are generally preferred from an environmental point of view but the use of conventional polyalkenes presents significant processing problems and they are not suitable for use in floor covering manufacturing facilities based on spread coating and calendering technology. In addition a particular problem in employing conventional polyalkene polymers in floor coverings, is that they do not provide the necessary physical characteristics required in the final product. In more detail floor coverings produced using conventional polyalkenes have been known to give insufficient tensile and tear strength, abrasion and stain resistance, and elastic recovery.
It is an object of the present invention to avoid or minimize one or more of the above disadvantages.
It has now been found that a particular class of polyalkenes, which are produced by single site catalysed polymerisation, can be successfully used in floor covering manufacture based on more or less conventional spread coating or calendering technology. More particularly, suitable polyalkenes in accordance with the present invention are those having a relatively narrow molecular weight distribution (MWD) and, a small amount of long chain branching and produced by single site catalysed polymerisation, and having the following characteristics:
a) Melt Index (MI) of from 0.1 to 100
b) Density of from 0.86 to 0.97; and
c) a DRI of from 0.1 to 6.0, preferably from 0.4 to 5.5.
As used herein the following terms have the meanings indicated:
Melt Index (MI) or I2 is the amount (in grams) of polymer resin which is extruded in a predetermined period of time (10 minutes) as measured in accordance with ASTM (American Standard Testing Method) D-1238 (190/2.16).
Molecular Weight Distribution (MWD) is the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (i.e. Mw/Mn).
Density is the mass (in grams) of 1 cubic centimeter of resin as measured in accordance with ASTM D-792 standard.
Dow Rheology Index (DRI) is an index of long chain branching measured by comparing the shift to the right (due to a longer relaxation time), relative to a polymer resin with zero long-chain branching (LCB), in a plot of zero shear viscosity against relaxation time (both from a cross viscosity equation).
Other abbreviations used herein which are common in the art include:
PHRxe2x80x94parts per hundred parts by weight of polymer resin (or principal polymer resin component).
Suitable polyalkenes in accordance with the present invention may also comprise a polyalkene having a relatively narrow molecular weight distribution (MWD) and, a small amount of long chain branching and produced by a single site catalysed polymerisation of at least one, linear, branched or cyclic, alkene having from 2 to 20 carbon atoms. Conveniently the polyalkene comprises a copolymer produced by copolymerisation of two or more alkenes comprising a first linear or branched, alkene having from 2 to 8 carbon atoms and, a second, linear, branched or cyclic, alkene having from 2 to 20 carbon atoms. This allows for greater design flexibility in relation to obtaining sheet materials with particular desired combinations of physical characteristics. In general there may be used up to 15 mole percent of said second monomer. It will of course be understood that where cyclic alkenes are used these may have more than one carbon ring and thus include bicyclic and tetra-cyclic alkenes such as norbornene and tetracyclododecene.
In another aspect the present invention provides a sheet material suitable for use in or as a floor covering and comprising a polyalkene resin in intimate admixture with one or more additives selected from a filler and a spread coating processing aid, wherein said polyalkene resin has a relatively narrow molecular weight distribution (MWD), preferably less than 3.0, and, a small amount of long chain branching and produced by single site catalysed polymerisation, and having the following characteristics:
a) Melt Index (MI) of from 0.1 to 100
b) Density of from 0.86 to 0.97; and
c) a DRI of from 0.1 to 6, preferably 0.4 to 5.5.
One of the very versatile features of metallocene catalysts is the range of comonomer which may be incorporated into polymeric chains by using such catalysts in the single site polymerisation of alkenes. Metallocene catalysts are, for example, capable of incorporating into polymer chains cyclic monomers, advantageously polycyclic monomers, including cyclic monomers such as norbornene (C7H10). Thus, for example it is possible to incorporate materials such as norbornene into copolymers with ethylene, which has the benefit of raising the toughness and melting point over conventional PE resins.
The new sheet materials provided by the present invention have the further advantage of suitability for incorporating various design features. It may be possible to incorporate graphic images into the flooring in a manner which will give an image with depth perception. Systems using ion projection technology are well known in the art. These systems use an electrostatic charge corresponding to the desired image. This image is deposited on the material with a drum or belt. The material bearing the electrostatic image is moved through a developer station where a toning material opposite charge adheres to the charged areas of the dielectric surface to form a visible image. Another layer of polymer may be deposited on top of this, and another image produced in this layer. By adding successive layers, each with its own image, it is possible to built a structure with an image depth perception. This art, using conventional resins, is explained in U.S. Pat. No. 5,347,296.
One advantage of using polymer prepared using metallocence-derived catalysts comes about during the image process. More particularly the use of metallocene catalysts permits the incorporation of boron containing end groups and/or very high levels of unsaturation. These end groups may be functionalized to provide additional means for facilitating imaging. Images may be created either via electrostatic projection systems or by functionalizing these end groups so the polymer chains will better combine with toner or pigments.
In another aspect the present invention provides a sheet material suitable for use in or as a floor covering and comprising a polyalkene resin in intimate admixture with at least one additive comprising a filler, wherein said polyalkene resin has a relatively narrow molecular weight distribution (MWD) and, a small amount of long chain branching and produced by single site catalysed polymerisation of a first, linear or branched alkene having from 2 to 8 carbon atoms and, preferably a second, linear, branched or cyclic, alkene having from 2 to 20 carbon atoms.
Whilst processing aids may be used in the new materials of the present invention, to adjust or accentuate particular processing characteristics such as reduced energy requirements and/or increased processing speed, it is a feature of the polyalkene resins used in the present invention that they do not require the use of a plasticiser thereby significantly reducing environmental problems caused by the migration of liquid plasticizers out of the material and/or loss of performance associated with the use of plasticisers.
Nevertheless, in those cases where it is desired to increase processability, then there may be used a processing aid or plasticiser, and it is an advantage of the present invention that a significantly smaller amount of plasticiser can be used as compared with polymer resins conveniently used in floor coverings. In a particularly preferred form of the invention there is, moreover, used a plasticiser or processing aid comprising a selectively polymerisable liquid monomer system which is substantially non-polymerisable under the sheet forming, e.g. extrusion, spread-coating or calendering, conditions used in the floor covering sheet material manufacturing process whilst being substantially polymerisable subsequently so as to produce a material substantially free of liquid plasticiser. In general the polymerisable monomer may be used in an amount relative to the polyalkene resin of from 20 to 80:80 to 20. Further details of suitable plasticisers are discussed hereinbelow.
In this connection it will be understood that there is normally used an initiator substance in order to induce polymerisation of the monomer and which is included together with the monomer in the monomer system. Accordingly in such cases it is important tha the initiator is one that is selectively activatable i.e. is substantially inactive under the polyolefin product forming conditions but may subsequently be activated under suitable plasticiser monomer polymerisation or curing conditions.
Various polyalkene resins suitable for use in the materials of the present invention are known in the art. In general they are produced by polymerisation of alkene monomers in the presence of particular catalysts which restrict the progress of the polymerisation and are known as metallocenes (the resulting polymers being commonly referred to as metallocene polyolefines conveniently abbreviated as MPOs). Such polyolefines and processes for their production are described in, inter alia, U.S. Pat. No. 5,272,236.
Preferred polyalkenes that may be mentioned comprise copolymers of ethylene and an alpha-alkene having from 4 to 20 carbon atoms, advantageously from 4 to 10 carbon atoms, for example propylene butene-1, or hexene-1, or a cyclic olefine such as norbornene; copolymers of propylene and an alpha-alkene having from 2 to 10 carbon atoms, for example butene-1, hexene-1, of a cyclic olefine such as norbornene; and copolymers of 4-methyl-1-pentene and an alpha-alkene having from 2 to 10 carbon atoms, for example, butene-1, hexene-1, or a cyclic olefine such as norbornene. Preferably there is used a copolymer containing up to 15 mole percent of comonomer. It will moreover be appreciated that there may be used more than one comonomer, that is, there may for example be used a terpolymer wherein are employed two different alpha-alkenes each having from 2 to 20 carbon atoms.
Suitable polyalkene resins that are commercially available from the Exxon Chemical company of USA and the Dow Chemical company of Midland, Mich., USA, are listed in Tables 1 and 2 below.
In a further aspect the present invention provides a polymer resin-based floor covering comprising at least one layer of a sheet material of the invention. It will be appreciated that in general such floor coverings comprise two or more different layers having particular functions, bonded together. Typically there may be included layers such as a foamed layer to provide cushioning; a structural layer comprising a reinforcing carrier or substrate impregnated and/or coated with a saturant formula; a solid backcoat layer; and a clear protective or topcoat layer.
For some types of applications little or no expansion in some or all layers of the floor covering structure will be required. The current invention includes a range of floor covering systems from those wherein all layers, except the topcoat, are foamed to those where none of the constituent layers are foamed.
The sheet materials of the invention may be produced by a process comprising the steps of:
providing a suitable polyalkene resin in accordance with the present invention and at least one additive comprising a filler and optionally a sheet formation, typically a spread coating or calendering, processing aid;
bringing said polyalkene resin into intimate admixture with said at least one additive in a high shear mixer for a period of at least 10 minutes at an elevated temperature of at least 75xc2x0, preferably from 100 to 250xc2x0 C., most preferably from 130 to 200xc2x0 C., for melting the polyalkenes and sufficient to bring the mixture into a substantially fluid state without substantial degradation of the mixture;
forming the fluid mixture into a sheet form; and
allowing said sheet to cool and solidify.
In one preferred aspect of the invention there is used a said fluid mixture which is substantially free of any plasticiser. Nevertheless, as discussed elsewhere herein, there may be included in the mixture one or more plasticisers or processing aids. Where there is used a polymerisable plasticiser, then the process includes further treatment of the solidified sheet in order also to solidify the plasticiser. Where a fugitive plasticiser is used the process advantageously includes the step of volatilizing said plasticiser.
The sheet material production processes of the present invention have significant advantages over those made using conventional polyalkene or polyolefin resins. Apart from the superior processability which allows the use of conventional existing production plant previously utilized for PVC resin based sheet materials with minimal modifications, they also have lower energy consumption costs due to the substantially reduced curing temperatures required as compared with PVC resin based production which involve increasing temperature to effect a thermal curing as opposed to a cooling to effect xe2x80x9ccrystallisation curingxe2x80x9d by xe2x80x9csolidificationxe2x80x9d. Further benefits that can be obtained in relation to particular floor covering layers in products of the invention include better toughness of the outer clear coat layer with better impact resistance resulting from the lower crystallinity associated with lower density; better cell recovery in foamed cushioning layers; and better filler acceptance due to more homogenous nature of the polymer (narrow MWD); and good flowability of the saturated layer resulting from high MI with little or no comonomer blocking.
In relation to the various aspects of the present invention it will be appreciated that other polymer resins outside those specified may be used in admixture with the specified ones e.g. in order to xe2x80x9cextendxe2x80x9d the specified polyalkene resin for reasons of economy by using a cheaper polyalkene resin, or to modify finish or other characteristics. The amount of such other polymer resin that may be used will depend primarily on how they affect the fluidity and spread coating characteristics of the materials of the invention. Thus for example there may be used up to around 50 to 60% w/w of said other polymer resin (relative to the total polymer resin) depending on the required use and properties of the sheet layer. Thus, for example, in relation to the clear coat layer, the amount of such other polymer resin would normally be restricted to a lesser amount of not more than around 15 to 20% w/w.
Additives that may be used in the materials of the present invention and the amounts thereof, will depend on the function and desired properties of the sheet material and may also, to some extent, depend upon the particular polymer resins used. Principal additives and additional processing steps generally well known in the art, that may be mentioned include the following:
1. Inorganic fillers and reinforcements can enhance the various polyolefin based layer or layers in the floor covering material, which is the subject of this invention. This enhancement can be through improvements in appearance, physical properties, or chemical characteristics. The particular inorganic filler/reinforcement attributes that are important are the nature of the inorganic material, the shape of the material, and any surface treatment or coating. There are many important aspects of the inorganic material. Density is important in the application and long term utility of a floor covering. Highly filled back coat layers (e.g. up to 85% by weight of filler) can be very useful in this regard. Another basic material attribute is hardness. Increased hardness is desirable in the final product, but too hard a filler (such as silica) can have negative effects on the wear of processing equipment, such as melt mixers and extruders. Table A lists some common inorganic fillers/reinforcers.
Whiting filler is used to increase opacity. Generally there is employed less than 500 PHR, preferably from 20 to 120 PHR in saturant formula and foamable cushioning materials and up to 200 PHR in solid backing layers.
The optical properties of titanium dioxide make it a particularly good pigment in obtaining a white colour with good opacity. Such a colour is desirable in the layer upon which the printed design is placed. This is located below the transparent wear layer. Lower levels of titanium dioxide (2 to 6 PHR) can be employed if a white filler such as Calcium carbonate is used at moderate levels in this layer.
Calcium carbonate is of particular utility in polyolefin based compositions. Hardness, stiffness, heat deflection temperature, slip resistance, stress crack resistance, weldability, printability, and antiblock characteristics are all improved. Thermal shrinkage and elongation, as well as water vapour and oxygen permeability are decreased.
Talc is another filler well suited to enhance polyolefin formulations for floor covering. It has a lamellar structure in contrast to the low aspect particulate structure of calcium carbonate. This lamellar form allows talc to be more effective than calcium carbonate with regard to increasing stiffness, heat deflection temperature and dimensional stability. The disadvantage of talc relative to calcium carbonate centre on reduced impact strength, matt surface, and lower thermooxidative stability. Mica also has a lamellar structure and has similar advantages and disadvantages.
High aspect ratio fillers/reinforcements such as wollastonite and glass fibres, have an even stronger effect than talc and mica on increasing the modulus of elasticity, tensile strength, and heat-distortion temperature of polyolefin based systems.
The improvements provided by high aspect ratio inorganic additives would be of particular assistance in these floor covering systems made using a permanent plasticizer or processing aid, such as liquid paraffin. In these cases, the stiffening action of such additives would compensate for the loss of stiffness produced by the liquid paraffin.
Silica in its fumed or precipitated forms can be useful at low levels (0.1 to 1.5%) in the polyolefin formulations where antiblocking and printability is of importance. In the floor covering system these would be in the wear layer and in the layer upon which the printed design is applied.
Alumina trihydrate and magnesium hydroxide, in the correct particle sizes which for most systems are less than 40 microns in diameter, can provide the same type of property enhancement provided by calcium carbonate. In addition, they can provide useful fire resistance and smoke control characteristics. This will be discussed in more detail in the fire resistance section.
2. Polyolefin materials for floor covering systems are enhanced by the use of the thermal and light stabilizers. For thermal stabilizers the amount and type that should be used will vary with the actual process used to fabricate the final structure. The melt spreader approach will provide a product having less heat history than either the melt calendering or extrusion routes. In all cases that involve foamed systems, however, the polyolefin resins will be exposed to temperatures over 180xc2x0 C. for some time during the process.
Suitable stabilisers include hindered phenol at from 0.05 to 0.30 PHR, optionally with co-stabilisers e.g. organosulphur compounds such as DSTDP at from 0.2 to 1.0 PHR. More particularly good thermal stability can be obtained in these polyolefin systems using a high molecular weight hindered phenol, such as Irganox 1010 from Ciba-Geigy, with one or more secondary antioxidants such as thioethers and phosphorus compounds. Distearylthiodipropionate (DSTDP) and Ultranox 626 from GE are examples of these types of materials. An effective thermal stabilizer package from such systems is 0.1% Irganox 1010, 0.1% DSTDP and 0.05% Ultranox 626.
Hindered amine light stabilizers (HALS) are particularly effective in protecting polyolefins from photo-oxidation. A Polymeric HALS, such as Luchem HA-B18 from Atochem, is particularly effective in its own right and has the added advantage of showing no antagonism for other additives such as DSTDP. The inclusion of 0.3% of Luchem HA-B18 in the outer wear layer and 0.15% in the layer just below the transparent wear layer will greatly enhance the light resistance of the subject polyolefin floor covering system.
3. Lubricants and processing aids may be of assistance in the manufacture of the polyolefin based flooring system. This will be very dependent on the specific process. For extrusion or melt calendering operations an external lubricant may be of assistance. Calcium and zinc stearate are appropriate as external lubricants. They also can provide some additional stabilization support. They can be added in the 0.1 to 1.0%, preferably 0.2 to 1.0% range is needed.
4. Depending on the spread coating or calendering process and conditions, melt strength enhancement of the polyolefin system may be useful. Grafts of polyolefins and acrylics are useful at the 0.1 to 1.0% range in proving a stronger more elastic melt.
5. In the polyolefin based floor covering which is the subject of this invention, for most applications it is desirable to have one or more of the layers in the structure (but not the wear layer) to be expanded in the form of a close cell foam. One effective route to such an expanded layer is through the use of a chemical blowing agent. In polyolefin systems azo compounds are especially effective. An example of this class of compounds is Azodicarbonamide (Celogen AZ from Uniroyal). A particularly useful feature of this compound is that its decomposition point can be reduced from 220xc2x0 C. to less than 170xc2x0 C. through the use of activators, such as zinc oxide. This activated system can be deactivated through the use of inhibitors such as benzotriazole. If inks containing benzotriazole are used to print on the surface of a polyolefin containing Celogen AZ and Zinc Oxide and the resulting structure, with a wear layer added over the foamable layer, is heated to temperature between the activated and inactivated decomposition temperatures, then a raised pattern (chemical embossment) is created in the sample.
A supplemental blowing agent such as aluminum trihydrate may be employed in these structures. Although its primary role is that of a flame retarding additive and inorganic filler it has a useful auxiliary role as a blowing agent in that it gives off water vapour when heated above 200xc2x0 C. A volatile fugitive processing aid or plasticizer can also have a useful role as a supplemental blowing agent.
In the case of azodicarbonamide this is generally used for foamable cushioning layers at from 2.0 to 4.5 PHR, together with a suitable foaming activator such as zinc oxide.
Some or all chemical blowing agents can be replaced with mechanical foaming, given the correct conditions. Such conditions involve the mixing into the polyolefin based mixture, that will become one of the layers in the floor covering material, air or another gas, under conditions that will produce the desired number and size of cells in the resulting foam. In the spread coating system the mixture as applied needs to have a foam structure near to that of desired product. In the extrusion or calendering process the gas needs to be in solution in the polymer or as small micro bubbles at the melt pressure in the extruder system. Expansion takes place as the melt leaves the extruder and goes from high pressure (100 to 700 PSI) to atmospheric pressure. In both cases, it is important for the cell structure to be frozen at the desired size by a rapid drop in the sheet temperature to below that needed for cell contraction or deformation.
6. The properties of the polyolefin structures in the subject floor coverings can be enhanced through the use of crosslinking, conveniently by means of an organic peroxide e.g. at from 0.1 to 5.0 PHR for increasing toughness and/or stiffness of the sheet layer. Dicumyl peroxide is a reagent used extensively for such reactions. This material becomes an effective crosslinking agent at 190xc2x0 C. In the case of crosslinked foamed polyolefin systems it is known that a better foam cell structure is developed if the crosslinking is done before the foam is formed. In systems involving Celogen AZ for foaming and dicumyl peroxide for crosslinking, both processes would take place at the same time and temperature. If a peroxide with a lower activation temperature, such as 2,2-bis(tert. butylperoxy) butane were used then the crosslinking could be carried out at about 170xc2x0 C. followed by a foaming process at 190xc2x0 C.
The development of strong crosslinked filled foam polyolefin systems can be further enhanced by treating the inorganic filler to be used with vinyl silane. The vinyl groups that become attached to the filler particles become active in forming the cross linked network initiated by the peroxide produced free radicals.
In non-expanded layers Dicumyl peroxide would be a good crosslinking agent. In layers to be expanded, using 2,2-bis(tert. butylperoxy) butane in conjunction with an activated Celogen AZ blowing system would be desirable. In all filled layers to be foamed, the filler should be treated with an agent such as vinyl silane that will provide sites of unsaturation on the filler particles.
7. The flammability and smoke generation of the polyolefin based floor covering system is of importance. Fire characteristics can be improved through a wide range of additives. Various inorganic compounds, such as aluminum trihydrate and magnesium hydroxide, that give off water at elevated temperatures are useful as dual fillers/flame retardants. Phosphorous compounds, borates, and zinc oxide all can play useful roles in improving the fire characteristics of polyolefin bases systems.
8. polymer resins other than the specified MPOs may be used as noted above as extenders or modifiers in amounts of from 10 to 30 PHR. Examples that may be mentioned include LLDPE (Linear Low Density PolyEthylene), EVA (Ethylene Vinyl Acetate), Ionomers e.g. SURLYN (TM) available from the DuPont Company, and VLDPE (Very Low Density PolyEthylene).
In addition, blends of two or more metallocene prepared polyolefins may be used to obtain particular combinations of desired properties.
To improve impact properties various types of elastomeric component additives can be used in generally known manner. These generally comprise small particles with a core of an elastomer e.g. butadiene or acrylic polymer coated with an outer shell that will provide good adhesion to the MPO polymer resin matrix. An example of such an elastomeric component core/shell modifier additive is Paraloid EXL-330 from the Rohm and Haas Company. This resin has an acrylate rubber core and a polymethyl methacrylate shell. Other types of modifiers that can be used to enhance impact properties include EPDM rubbers, such as Polysar manufactured by Bayer; A/B/A block copolymers, such as Kraton manufactured by Shell; and multiple domain elastomer systems, such as those described in European Patent No. 583,926.
9. Other additives that may be mentioned include dyes, inks, antioxidants etc. which are generally used in relatively small amounts at less than 50 PHR. Antistatic characteristics can also be important for some applications.
In this case, the use of various internal antistatic agents in the wear layer would be appropriate. Many antistatic additives are compounds with hydrophilic and hydrophobic sections. A common material of this type is a mono ester of a polyol, such as glycerol, with a long chain fatty acid, such as stearic acid. The polyol portion is very polar and would come to the surface of a polyolefin, while the fatty acid is xe2x80x9cpolyolefin-likexe2x80x9d and would stay within the plastic. 9. The hydrophilic part can be cationic, anionic, or nonionic. Levels of 0.1 to 0.5 PHR in the outer layer of the structure are appropriate.
10. Carriers or substrates used with saturant formulations may have various forms e.g. woven or non-woven mesh or fabric, or tissue, of more or less thermally stable materials such as glass fibre.
The polyalkene or polyolefin resins used in accordance with the present invention may be of various different types including random bipolymers and terpolymers, and block copolymers, based on a variety of monomer units including lower alkene, preferably 1-alkene, having from 2 to 8 carbon atoms e.g. propylene but most preferably ethylene; dienes; cycloalkenes; and vinyl aromatic compounds.